This longitudinal study of prospective screening of a large kindred was started in 1971. Its purposes are (1) to develop the most sensitive methods for the early diagnosis of thyroid C-cell and adrenal medullary neoplasia, and to deter-mine the impact of prospective screening for medullary thyroid carcinoma and pheochromocytoma on the natural history of MEN-2a. We have determined the genetic defect in our family and now are using PCR of the involving DNA to make the diagnosis of gene carriers.